Sustained release dosage forms can decrease the frequency of administration of biologically active compounds and can also serve to reduce side effects by reducing peak serum levels of the compounds. There is also a significant advantage in administering biologically active proteins and polypeptides in sustained release dosage forms, since compounds of those classes generally have short biological half-lives.
Aqueous gels of pharmaceutically acceptable polymers such as gelatin, methylcellulose and polyethylene glycol have been used to control the release rate of drugs from dosage forms. The diffusion of drugs through such gels is hindered by the viscosity of these systems as well as the tortuous diffusion path that results from the three dimensional polymeric network that is present. These gels cannot easily be used to sustain the release of drugs administered parenterally due to the inherent problem of injecting such viscous materials through a hypodermic needle. In addition, the high molecular weight of these polymers prevent their rapid elimination from the injection site.
Lecithin gels are known per se, see e.g., Scartazzini et al., J. Phys. Chem., 92:829-833 (1988) and Luisi et al., Colloid Polym. Sci., 268:356-374 (1990). These gels are formed ex vivo by the addition of a critical amount of water to a mixture of lecithin and an organic solvent for the lecithin. Lecithin gels have many of the rheological properties of polymeric gels.